A. Field of the Invention
The present invention relates to a flat color cathode ray tube (hereinafter referred to as "CRT"). More particularly, it relates to a shadow mask for a flat CRT that prevents an electron beam from mislanding when kinetic energy of the electron is converted into heat energy when colliding with the shadow mask, and then the mask expands with heat during operation of the CRT.
B. Description of the Prior Art
As shown in FIGS. 1 and 2, a conventional flat CRT comprises a safety glass 1 bonded to the front of a flat panel 2 with a resin in order to maintain an explosion-proof characteristic; a funnel 3 attached to panel 2 by a glass frit; an electron gun 5 being encased in the neck 3a of funnel 3 for emitting R, G, and B electron beams 4; a shadow mask 7 behind panel 2 and having innumerable slit-shaped apertures for selecting the color of the electron beam; and a frame 6 supporting in order to maintain a constant distance between the shadow mask 7 and the panel 2.
As illustrated in FIG. 3, the shadow mask 7 has an effective face area 10, which is the electron beam landing area; and an area 11, where apertures are not formed; and an ineffective face area 12, which is not used in practice. Line 13 refers to the position where frame 6 is attached.
The electron beams 4, emitted from the electron gun 5, pass through electron beam apertures 7' in the shadow mask 7, before colliding with a phosphor screen (not shown) applied to the inner surface of the panel 2. Kinetic energy of the electron beams 4 causes multiple phosphors to emit light so that an image is displayed on the panel 2. Only 20% of the electrons pass through the electron beam apertures 7' of the shadow mask 7, the rest collide with shadow mask 7, and then are converted into heat so that the shadow mask 7 expands from the heat. Such a phenomenon is called "doming."
The position where the electron beams land on the phosphor screen is changed by the doming phenomenon, and therefore degradation of color purity is caused. In order to solve this problem, an INVAR mask, which has a low coefficient of thermal expansion has been used. Another approach is to use a bimetal as a spring for fixing the shadow mask at the panel. However, the cost of manufacture is increased, and operation efficiency is deteriorated.
A flat foil tension mask (hereinafter referred to as "tension mask") is designed to enhance definition in recent years. Because tension is applied to the tension mask, it compensates for expansion due to heat caused by electrons colliding against the mask. As a result, the position where the apertures are formed in the mask is not changed as even at high temperature.
The tension mask is assembled inside the front of the CRT, adjacent to the flat panel, and is fastened to a support fixing the mask so that electrons land on their geometrically intended phosphor dots on the inner surface of the panel. The tension mask, with a thickness of 0.025 mm is disposed at a fixed distance from the inner surface of the panel, and is a color selection device for selecting the electron beams and causing the phosphor screen, coated with red, green, and blue phosphors, to emit light in accordance with the corresponding signal.
Such a tension mask has dot-shaped electron beams apertures, and thus, Young's moduli in the horizontal and vertical directions are about the same. As a result, the effective face area can be uniform and considerable tension can be applied.
Recently, a mask with slit-shaped apertures has been employed to enhance screen characteristics such as definition, wave patterns on the screen, and purity redundancy. However, there is a large difference in Young's moduli between the horizontal and vertical directions, and the effective face area is not uniform and less tension can be applied. If little strain is applied, the electron mislands due to expansion of the mask. If much strain is applied, stress is concentrated on certain regions of the mask so that the plastic deformation occurs, deforming or breaking the mask.
In order to solve these problems, apertures are formed around the circumference of the effective face area of the mask. As shown in FIG. 4, the electron beam apertures are formed in the ineffective face area A in order to produce anisotropic elasticity. However, if much strain is applied, stress is concentrated in corner B of the ineffective face area, reducing the amount of tension that can be applied.